Batch register restore on MSHV and WHP

[ludfjig]

Batch general, debug, special, XCR0, and MSR writes during x86_64 vCPU restore on MSHV and WHP.

benchmarks (vs main branch):

MSHV:

snapshots/restore/default
time: [83.550 µs 84.031 µs 84.443 µs]
change: [−29.780% −28.618% −27.365%] (p = 0.00 < 0.05)
Performance has improved.

snapshots/restore/small time: [91.355 µs 92.208 µs 93.294 µs]
change: [−24.927% −22.241% −19.535%] (p = 0.00 < 0.05)
Performance has improved.

snapshots/restore/medium
time: [105.37 µs 105.78 µs 106.27 µs]
change: [−28.331% −21.040% −12.627%] (p = 0.00 < 0.05)
Performance has improved.

snapshots/restore/large time: [167.54 µs 169.86 µs 172.89 µs]
change: [−31.405% −9.3155% +20.327%] (p = 0.53 > 0.05)
No change in performance detected.

WHP:

my windows machine is too noisy to measure

KVM:

no difference

[Copilot]

Pull request overview

This PR reduces x86_64 snapshot-restore overhead by batching general/debug/special/XCR0/MSR register writes into a single backend call when running on MSHV or WHP, avoiding multiple hypercalls during vCPU reset.

Changes:

• Extend the VirtualMachine trait with a batched-register write API and plumb new RegisterError variants for it.
• Implement batched register restore for WHP and MSHV, with KVM/HVF reporting the capability as unsupported.
• Fold MSR restore into HyperlightVm::reset_vcpu and update sandbox restore to pass snapshot MSRs into the reset path.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/hyperlight_host/src/sandbox/initialized_multi_use.rs	Pass snapshot MSRs into reset_vcpu so restore can batch state updates.
src/hyperlight_host/src/hypervisor/virtual_machine/whp.rs	Add WHP batched register write implementation via set_registers.
src/hyperlight_host/src/hypervisor/virtual_machine/mshv/x86_64.rs	Add MSHV batched register write implementation via set_reg.
src/hyperlight_host/src/hypervisor/virtual_machine/mod.rs	Define the batched-register capability + API on the backend trait and add new error variants.
src/hyperlight_host/src/hypervisor/virtual_machine/kvm/x86_64.rs	Explicitly mark batching as unsupported on KVM.
src/hyperlight_host/src/hypervisor/virtual_machine/hvf/mod.rs	Explicitly mark batching as unsupported on HVF.
src/hyperlight_host/src/hypervisor/hyperlight_vm/x86_64.rs	Use the new capability to batch reset writes and incorporate MSR restore into reset.
src/hyperlight_host/src/hypervisor/hyperlight_vm/test_support.rs	Forward the new trait methods through the fault-injecting VM wrapper.

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